TRANSPORT IN PLANTS
; Describe the structure of xylem vessels, sieve tubes and companion cells.
Vascular tissue transports substances around plants. They are made of xylem tissue and phloem tissue.
These are composed of bundles of xylem vessels which are long, extremely narrow, hollow tubes formed from columns of elongated cells. The tubes/ vessels are made from dead cells thickened with lignin (a woody material). This is formed as the cross walls between adjacent cells in a column die, the cell contents die and the cell walls become thickened with tough lignin in spirals, rings or reticulate patterns.
Functions of Lignin:
1. Strengthens cell walls.
2. Helps make the vessels waterproof.
; Wood is almost entirely lignified xylem.
These are composed of bundles of phloem sieve tubes which are long, narrow tubes with perforated sieve plates along their length. Sieve plates are formed from columns of elongated cells by the cross walls between adjacent cells becoming perforated with small holes. Each cell is called a sieve tube element and contains living cytoplasm but no nucleus and has a companion cell next to it. The companion cell contains a nucleus which controls the functioning of both cells.
Note that unlike xylem tissue, phloem tissue is living.
; Explain how the structure of xylem vessels suits them for their function.
Xylem vessels transport water and minerals upwards from the roots to the leaves of plants. The lignin in xylem strengthens the cells walls for transport of water and it
also helps to make the xylem vessels waterproof. The strong cell walls are then able
to give rigidity, strength and support to the plants’ stems. The long, narrow and
hollow tubes of the xylem allow water to pass through freely.
The diagram below can assist you with the transpiration notes I gave you in class, dealing with root pressure, capillarity etc….
Minerals enter the plant through root hairs via active transport. In root tissue the
concentration of mineral ions is higher than in the soil. Hence active transport occurs as this happens from low to high concentrations. Active transport in this case, also allows cells to build up stores of substances that would otherwise be spread by
diffusion. A lot of energy is required for the process and this energy comes from the mitochondria in the root hair cells. The minerals that enter the root cells then dissolve in water and are transported via the xylem to the rest of the plant.
; State the function of phloem in the transport system of plants.
Phloem transport food materials, mostly in the form of sugars, up and down the stem to all parts of the plant. The companion cells and sieve cells work together to transport sugars to where they are needed in the plant. Translocation is the
movement of sugars and other substances from one region to another through the sieve cells.
Sugars move from the leaf cells to the sieve cells by active transport. In the sieve
tubes, the sugar and other substances are carried to the parts of the plant as needed. Diffusion carries sugars from the leaves (high conc) to the roots (low conc). Sugars are converted to starch and stored in the roots.
MAKE YOUR OWN NOTES ON THE EVIDENCE THAT EXISTS TO SHOW
THAT PHLOEM TRANSPORTS FOOD AND ALSO ON HOW TO USE THE
POTOMETER TO MEASURE RATE OF PHOTOSYNTHESIS. (To show that
phloem transport food, look at aphids and radioactive tracers, and tree ring experiments).